EVBEC vs chrony

[SpudFarm]

today i got my chrony and tested how good the EVBEC online is.

at a 5x mix with a marble i got 1075 FPS so the EVBEC was of with 109.5 FPS something i don't think is so bad... if i have measured the chamber and barrel very accurate it may have hit better.

i just want to let you know how accurate it is.

my pneumatic was at 690 FPS and the ggdt told me 800 so that was really off...

so what do you think about the numbers?

[bluerussetboy]

so both were off ~110 FPS, but with using different programs? interesting to say the least. i'm not a math geek, but i would start to question the accuracy of your chronograph setup and your actual gun measurements.

So was your results an average of 10 shots with the high and low removed? or something to that effect? or was your results just the high?

More info about how you ran your tests is definitely needed to accurately compare everything(EVBEC,GGDT and chrony).

[CS]

Hey thanks for testing out the numbers. Always wondered just how accurate the program was.

From what I understand BLB made the program to give relatively accurate numbers, however nothing definitive. [From documentation, conversation, and the many other variables not covered in EVBEC]

Now to your assessment of GGDT. Your numbers were probably off simply because the amount of variables, and there difference in reality versus calculation. Mr. Hall allows for more variables to be manipulated, so many are assumed, or not even verified by the user.

If I understand correctly EVBEC uses calculations based off of real data probably allowing it to better guesstimate the unlisted variables. GGDT on the other hand generates its numbers from cold hard math. (pretty sure on that) So ideally GGDT is better, however EVBEC may render better results given our limited knowledge of the variables, and other effects. Mind you EVBEC, and GGDT are used for calculating two different methods of propulsion - pneumatic, and combustion.

Thanks again. Excuse my rambling as well.

[DYI]

I agree with Pimp - I don't think that GGDT was that far off. You probably screwed up a variable somewhere, or your instruments weren't reading properly (gauges, chrony, etc...). You'd have to ask D_Hall about that to be sure though.

[SpudFarm]

i don't remember hom much the ggdt said on the pneumatic but it was of ALLOT.

i got a chrony beta and did one shot from the hybrid (started to get dark) and it measured 1075 FPS or 327,7 m/s.
i shot at a dictance of 3feet with a marble at 5x with a 80in long barrel.
the pipe i used as chamber is 2" by 8" but is 12" with all the couplings.

i think the ggdt was of by 146 fps.

i used the same barrel on booth cannons and the burst disk in the hybrid was 5layers of heavy tape. (4layers doesn't burst at 3x) and it did not burst so clean.

more info needed then ask!

Edit: the posts just falling in:D
the ggdt was of so far because of me. i got some help of galfisk with the numbers but there is no human that are perfeckt in this world...
i may do some tests for you if you think something is wrong...
just tell me what can have been wrong and i will do one more test tomorrow

thanks to all of you !

[DYI]

Have you done a liquid volume measurement on the pneumatic to be sure of its chamber volume? What about the hybrid?

How accurate is the gauge on your pneumatic? With such an absurdly light projectile as you're using (3 grams), even a slight variation in pressure (in the pneumatic) or mix (in the hybrid) from what you think you're using could throw your results really far off.

How aerodynamic is your projectile? For such a light weight, it could lose some velocity from the muzzle to the chrony if it isn't streamlined.

How accurate is the chrony?

Could you show me a screenshot of the GGDT model, and give a link to the pneumatic?

[SpudFarm]

this is the pneumatic used
if you can you may run it through the ggdt for me at 130psi if you get the specs needed on the topic...

i don't think it change velocity on 3feet. the pneumatic don't have supersonic muzzle blast so i shot that at point blank. when i used the ggdt i plotted in 5gram protectile weight.

ask for specs needed if you don't got what you need.

(my compressor turns on at 8bar and of at 10 so i used the 9bar at a 4shot av)

[boilingleadbath]

Interesting; even without a fan, EVBEC predicted better-than-actual performance.
It seemed that for the longest time, the assumption was that filling the chamber with air was roughly equivalent to the use of a chamber fan.

Anyways, I'm not too upset with that prediction either - both the high-velocity and hybrid aspects of the system are rather iffy. Especially the high-velocity stuff.

That said, a couple of factors that EVBEC doesn't take into account which may have mattered here:
1) Flow loses. You have a very longer barrel than was typical in the tests, and a rough burst-disk rupture.
2) There may have been some leakage around the projectile
3) It's hard to push air through a tube at higher than (or very near) the speed of sound, so backpressure in front of the projectile will be higher than EVBEC pretends it is. (heck, EVBEC assumes the speed of sound in a gas is infinite!)

[DYI]

heck, EVBEC assumes the speed of sound in a gas is infinite!

Well there's your problem

And I think that the reason behind the hybrid results being higher than they should is that, while incoming air probably does mix the fuel, it doesn't create the turbulence that a fan does, which really negates half of the fan's function. Has anyone ever actually put a fan in hybrid to see if it increases performance?

[Fnord]

Has anyone ever actually put a fan in hybrid to see if it increases performance?

On it...

This should be fun
Fun meaning a pain in the ass

[DYI]

I might try a fan in mine, but I'd have to get another two holes tapped into the back plate, and the metering and ignition equipment won't be arriving until early April. If you have any success, it would certainly be worth trying though (and because we all know how fun it is to tap threads into 1-3/4" hardened steel plate...

[Fnord]

I'll probably get the fan installed within a week.
I don't have a chrony, but any performance increase worth having will show up with increased kick or penetration depth. If I can't tell the difference, it's probably not worth it.

You can use one of the wires from your ignition circuit as one side of the fan circuit, so you'll only have to tap one extra hole (you won't need to tap anything if you aren't using the chamber wall as part of the ignition circuit; you can run the + fan circuit through one spark gap wire and use the chamber wall as the - ).

Also, I have a question to anyone:
How might I go about protecting the fan from such an environment? I have used PVC as as a spark gap separator and it became charred black. And not because of carbon deposits either; the pvc itself was being burned.

[SpudFarm]

i don't think you can use a chamber fan in a hybrid... there are some extreme temperatures and it would start to melt it on the first shot.

[DYI]

That's why I'm not taking the challenge first - I don't know how well the fan would hold up. I too have seen some interesting effects on things left in hybrid chambers. And how would I use the same wire for ignition and a fan? I don't know a lot about electronics, but wouldn't the voltage spike from ignition harm a running fan?

[Fnord]

You can use the same wire from the high voltage circuit in the same way you can use the ground outside as part of a fan circuit.

Text doesn't work well. picture: